The invention concerns a process for the pyrolysis of a hydrocarbon feed containing at least two carbon atoms which is carried out simultaneously with decoking of the coke deposited on the reactor walls.
It is of particular application to the continuous production of acetylene or acetylenic compounds such as methyl acetylene.
In processes for the high temperature thermal transformation of hydrocarbons containing at least one carbon atom, for example pyrolysis between 90.degree. C. and 150.degree. C. or steam cracking at about 850.degree. C. near the end of the heating zone, coke forms and deposits on the surface of the reactor walls. The reactor is then decoked, normally using an air/steam mixture at temperatures which are usually below 90.degree. C., attempting in the case of metal furnaces to avoid any overheating or hot spots which could damage the metal tubes of the furnace. Exothermic decoking thus requires the whole unit to be shut down and in particular, it requires the furnace to be disconnected from the downstream heat exchangers, reducing the total productivity of the unit. Further, safety regulations require the hydrocarbon introduction lines to be disconnected and replaced by air introduction lines, thus requiring a very long downtime for the unit.
The same disadvantages are there when rebuilding the unit for the pyrolysis phase, with the additional necessity of purging the reaction zone and lines with an inert gas.
Pyrolysis of hydrocarbons containing at least two carbon atoms to produce olefinic or acetylenic hydrocarbons has been described, in particular in our French patent application FR-A-2,715,583 (U.S. Pat. No. 5,554,347), European patent EP-A-0 733 609 and FR 95/15527, which are hereby incorporated by reference.
The prior art is illustrated by patents EP-A-0,542,597 and FR-A-1,501,836.
Pyrolysis reactors of ceramic material have been used in which non impermeable walls which are advantageously of ceramic material determine channels in which the feed and the reaction effluents circulate. These walls advantageously have a shape which is adapted to create turbulence and which, for example, comprise cells or cavities about the heating means. These latter are generally sleeves containing an electric heating element or a gas burner.
These high technology reactors, however, require high investment costs and their energy requirement, in particular of electrical energy, results in high operating costs. European patent application EP-A-0,733,609 describes the possibility of using a steam cracking effluent, as a feed for the pyrolysis reactor as it already contains unsaturated hydrocarbons. The energy required to convert the feed to acetylenic would then be greatly reduced.
However, industrial steam cracking must be stopped every two to three months for decoking. A pyrolysis furnace operating at a higher temperature must be decoked more frequently, for example every four to five days. During the decoking step, the furnace must be isolated. Unfortunately, sealing valves which operate between 800.degree. C. and 900.degree. C. do not exist. An alternative thus consists of sending the effluent from the steam cracker to the pyrolysis furnace, following cooling by passage through a transfer line exchanger, but the benefit of using hot gas is lost and the gain is thus small. Further, the dead volume of the transfer line exchanger encourages secondary reactions to the detriment of the yield of ethylene.
A further disadvantage is linked to the frequency of tube decoking, every two to three months. At the end of a cycle, the inside of the tubes is covered with a thick layer of coke. Coke tends to detach itself at times and is entrained by the gas stream at speeds which are of the order of 200 m/s, risking damage to the ceramic material sleeves in the pyrolysis furnace downstream of the steam cracking furnace.